Microvalves have been fabricated from monolithic polymer materials for use in controlling fluid flow in microfluidic systems. These microvalves are typically fabricated by photoinitiating phase-separated polymerization in specified regions of a three-dimensional microstructure that can be of glass, silicon, or plastic. The valve function is achieved by controlling the shape of the polymer monolith and by designing the monolith to move freely within microfluidic channels. Measurements of the pressure required to actuate these polymer microvalves clearly indicate that for smooth channel walls the force requirements are proportional to an effective friction coefficient between the polymer monolith and the channel walls. Consequently, reducing the coefficient of friction at the substrate or channel wall-polymer monolith interface can reduce actuation forces.
The coefficient of friction has two components that are a function of: 1) the deformation of the polymer monolith caused by small (typically μm-size) geometric irregularities in the channel wall; 2) intermolecular interactions between the channels walls and the surface of the polymer monolith. In the prior art, provision for intermolecular interactions was made by appropriate selection of charged moieties in both the mobile polymer monolith and channel wall modifications such that the polarity of charge in both these components was the same, thereby eliminating electrostatic interactions. However, selection of appropriate charged moieties can present fabrication difficulties and the prior art did not address changes in surface energy of uncharged species to effect reduction in the coefficient of friction between channel walls and the mobile polymer monolith. Moreover, there is no provision in prior art for reducing or eliminating deformation of the mobile polymer monolith. A comprehensive discussion related to the manufacture of monolithic polymer microvalves and their use in microfluidic systems is contained in prior co-pending application Ser. Nos. 09/695,816, filed Oct. 24, 2000 and 10/141,906 filed May, 09, 2002, incorporated herein by reference in their entirety.